A few dozen Muslims gathered at the towering Gothic cathedral in Rouen, near Saint-Etienne-du-Rouvray where the 85-year-old Rev. Jacques Hamel had his throat slit by two teenage Muslim fanatics on Tuesday.
"We are very moved by the presence of our Muslim friends and I believe it is a courageous act that they did by coming to us," Dominique Lebrun, the archbishop of Rouen, said after the service.
Some of the Muslims sat in the front row, across from the altar. Among the parishioners was one of the nuns who was briefly taken hostage at Hamel's church when he was killed.
"Look at this whole Muslim community that attended Mass," she said. "I find this very heartwarming. I am confident. I say to myself that this assassination won't be lost, that it will maybe relaunch us better than politics can do. Maybe we will react in a better way."
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Many of the Muslims who attended the service in Rouen - including those with the banner - were Ahmadiyya Muslims, a minority sect that differs from mainstream Islam in that it doesn't regard Muhammad as the final prophet.
At Paris' iconic Notre Dame cathedral, Dalil Boubakeur, the rector of the Mosque of Paris, said repeatedly that Muslims want to live in peace.
"The situation is serious," Boubakeur told BFMTV. "Time has come to come together so as not to be divided."
In Italy, the secretary general of the country's Islamic Confederation, Abdullah Cozzolino, spoke from the altar in the Treasure of St. Gennaro chapel next to Naples' Duomo cathedral.
Three imams also attended Mass at the St. Maria Church in Rome's Trastevere neighborhood, donning their traditional dress as they entered the sanctuary and sat down in the front row.